This invention teaches novel superabsorbent filter media, a fuel filter using such filter media and a method of manufacturing such filter media. The fuel filter media is suitable for use in manufacturing accordian pleated filter cartridges at high production rates and low cost by using a rotary pleater or scoring machine rather than slower blade-type pleaters. The filter medium is a thin laminate of filter paper, superabsorbent resin powder, and tissue paper, rolled together under pressure and bonded with water vapor. Unlike other commercially available superabsorbent filter media laminates, this construction is thin and flexible enough to be rotary pleated at low cost.
Diesel fuel, jet fuel, and, to a lesser extent, gasoline dissolve and entrain small amounts of water. The water may be introduced by leakage, accidental contamination, or, most commonly, by atmospheric condensation. Hydrocarbon fuels can dissolve about 75 to 150 parts per million (ppm) of water at room temperature. Water solubility increases about 1 ppm for each 1.degree. F. rise in temperature, and decreases about 1 ppm for each 1.degree. F. fall in temperature. Excess water or undissolved water may accumulate in lower parts of a fuel handling system as a result of temperature changes over a period of time.
Undissolved water can be harmful to engines in which the fuel is used. Diesel engine injectors may be damaged by steam formation, jet turbine engines may flame-out, and gasoline engines may suffer ignition problems. Many water separating devices and filters have been developed to remove undissolved or free water from these fuels and prevent these engine problems.
Simple mechanical devices based on separation by gravity or centrifugal force are satisfactory if the free water is present as a discrete second phase. However, free water is often emulsified by pumps and valves, and may remain as a stable emulsion, especially in diesel or jet fuel. Two-stage coalescer/separators are designed to remove water emulsions. The coalescer breaks the emulsion by preferential wetting of fibrous materials such as fiber glass. The water is accumulated into large droplets and is removed by gravity separation against a hydrophobic separator material such as Teflon coated wire cloth or silicone impregnated paper. The presence of wetting agents or surfactants may interfere with the coalescence of water emulsions, especially in jet and diesel fuels.
More recently, superabsorbent materials have been used to remove water from fuels directly during filtration to remove dirt and similar contamination. The water absorbent material is pleated with the filter paper or is added as a lamination to the filter paper. Water is absorbed from the fuel in both droplet and emulsion form. Unlike coalescers, absorbent filters are not disarmed by surfactants. They are called "superabsorbent" filters because the absorbent materials can hold from 30 to more than 300 times their weight in water.
Many superabsorbent materials were first developed for use in personal care products such as tampons, disposable diapers and panty liners, and were later adapted to filters. Carboxymethyl cellulose (CMC) holds about 30 times its weight in water and was manufactured for use as a tampon material. It was available in several grades of paper-like material, and was incorporated in several commercial superabsorbent filters for fuel applications. Since toxic shock syndrome problems were traced to the CMC tampon, the product has not been made in the United States.
Other superabsorbent materials such as starchacrylonitrile copolymer (or graft), polyacrylamide, and polyacrylic acid resins have been produced in powder form for personal care applications, and have been adapted for use in superabsorbent filters. These polymers hold several hundred times their weight in water, and are used by distributing a small amount of powder into a fibrous matrix such as fiber glass or non-woven polyester fiber mat. The fibrous material is bonded to a filter paper using water vapor to wet the resin which acts like a glue, without consuming much water absorbent capacity. These superabsorbent filter media are now commercially available.
A major application for superabsorbent filters is on fuel dispensing pumps. Spin-on filters, similar in construction to spin-on lube oil filters, are installed by the manufacturer within the dispensing pump housing, or can be retrofitted using an adaptor head at the pump/hose coupling. Such superabsorbent filters remove both water and dirt from gasoline and diesel fuels with good filtration and long service life. Intermittent, small amounts of water are absorbed, and later released by dissolving harmlessly in dry fuel. Accidental large slugs of water are intercepted, preventing damage to vehicle fuel systems and engines, and dispensing flow is automatically shut down to insure detection and correction of the problem.
Presently available superabsorbent media and superabsorbent filter media laminates are fairly thick to ensure sufficient contact time for water absorption during flow. As a result, the number of pleats and the total filter area in a pleated cartridge assembly are limited. This, in turn results in a higher fluid velocity or flow rate per unit filter area, and partially defeats the original intent to provide sufficient contact time for absorption. Moreover, the thicker materials must be pleated using a blade-type pleating machine such as a Chandler or Rabofsky pleater, rather than a high-speed rotary or scoring machine pleater. An additional wrapping of superabsorbent media around the internal support core is often used to intercept water leaks through the pleated media, and to aid in shut-down.
The thicker pleats are widely spaced at the outside diameter to allow flow without pinching off at the inside diameter of the pleat block assembly. Automatic shut down of fuel flow in response to accidental ater slugs depends on absorption of water by the superabsorbent material and swelling of the composite or laminate. With sufficient swelling, the pleats are squeezed together, first at the inside diameter, but only later at the more widely spaced outside diameter. A "belly band" or open mesh wrapping is often added to the pleated filter element assembly to constrain bulging and more effectively shut down fuel flow. This adds cost for materials and labor.